Heterologous surface display of proteins (Stahl and Uhlen, TIBTECH May 1997, 15, 185–192) on recombinant microorganisms via the targeting and anchoring of heterologous proteins to the outer surface or the cell wall of host cells, such as yeast, fungi, mammalian cells, plant cells, and bacteria, has been possible for several years. Display of heterologous proteins at the surface of these cells has taken many forms including the expression of reactive groups such as antigenic determinants, heterologous enzymes, single-chain antibodies, polyhistidyl tags, peptides, and other compounds. Heterologous surface display has been applied as a tool for applied and fundamental research in microbiology, molecular biology, vaccinology and biotechnology. Another application of bacterial surface display has been the development of live-bacterial-vaccine delivery systems. The cell-surface display of heterologous antigenic determinants has been considered advantageous for inducing antigen-specific immune responses in live recombinant cells used for immunization. Another application has been -the use of bacterial surface display in generating whole-cell bioadsorbents or biofilters for environmental purposes, microbiocatalysts, and diagnostic tools.
Generally, chimeric proteins include an anchoring or targeting portion that is specific and selective for the recombinant organism, wherein the anchoring portion is combined with the reactive group, such as the antigenic determinant, heterologous enzyme, single-chain antibody, polyhistidyl tag, peptide, or other compound. A well known anchoring portion comprises the so-called LPXTG (SEQ ID NO: 1) box, which covalently binds to a Staphylococcus bacterial surface, i.e., in the form of a fully integrated membrane protein. In this manner, at least two polypeptides of different genetic origins may be joined by a normal peptide bond to produce a chimeric protein. For example, PCT International Patent Publication No. WO 94/18330, which relates to the isolation of compounds from complex mixtures and the preparation of immobilized ligands (bioadsorbents), discloses a method for obtaining a ligand comprising anchoring a binding protein in or at the exterior of a cell wall of a recombinant cell. The binding protein is essentially a chimeric protein produced by the recombinant cell and includes an N-terminal part derived from an antibody that is capable of binding to a specific compound, wherein the N-terminal part is joined to a C-terminal anchoring part, derived from an anchoring protein purposely selected for being functional in the specific recombinant cell chosen. PCT International Patent Publication No. WO 97/08553 discloses a method for selectively targeting proteins to the cell wall of Staphylococcus sp., using anchoring proteins which include long stretches of at least 80–90 amino acid long amino acid cell-wall-targeting signals. The signals are derived from the lysostaphin gene or amidase gene of Staphylococcus and encode for proteins that selectively bind to Staphylococcus cell-wall components.
Vaccine delivery or immunization systems with attenuated bacterial vector strains that express distinct antigenic determinants against a wide variety of diseases are currently being developed. Mucosal vaccines for nasal or oral passages using these attenuated bacterial vectors have received a great deal of attention. For example, both systemic and mucosal antibody responses against an antigenic determinant of hornet venom have been detected in mice orally colonized with a genetically engineered human oral commensal Streptococcus gordonii strain that expresses the hornet venom antigenic determinant on its surface (Medaglini et al., PNAS 1995, 2; 6868–6872). A protective immune response was also elicited by oral delivery of a recombinant bacterial vaccine that included tetanus toxin fragment C constitutively expressed in Lactococcus lactis (Robinson et al., Nature Biotechnology 1997, 15; 653–657). Mucosal immunization is considered an effective means of inducing IgG and secretory IgA antibodies directed against specific pathogens of mucosal surfaces.
Immunogens expressed by bacterial vectors may be presented in a particulate form to antigen-presenting cells, such as M-cells, of the immune system and therefore should be less likely to induce tolerance when compared to soluble antigens. Additionally, the existence of a common mucosal immune system permits immunization of one specific mucosal surface in order to induce secretion of antigen-specific IgA and other specific immune responses at distant mucosal sites. A drawback to using bacterial vectors for immunization is the potential of the bacterial strain causing inflammation or disease and potentially leading to fever or bacteremia. Instead of using attenuated bacterial strains that may become pathogenic, recombinant commensal bacteria, such as Streptococcus sp. or Lactococcus sp., may be used as vaccine carriers.
A potential problem with recombinant commensal microorganisms is that they may colonize the mucosal surfaces and generate a long term exposure to the target antigens expressed and released by the recombinant microorganisms which may cause immune tolerance.
Additionally, the use of genetically modified microorganisms that contain recombinant nucleic acid has met considerable opposition from the public as a whole, stemming from a low level acceptance of products which contain recombinant DNA or RNA. Similar objections exist against even the use of attenuated pathogenic strains or against proteins, or parts of proteins, derived from pathogenic strains. Further, the heterologous surface display of proteins described herein entails the use of anchoring or targeting proteins specific and selective for a limited set of microorganisms, which are of recombinant or pathogenic nature which greatly restricts their potential applications.
The protein anchor of L. lactis, AcmA (cA), its homologs and functional derivatives (PCT International Patent Publication No. W099/25836) bind in a non-covalent manner to a wide variety of Gram-positive bacteria. Binding also occurs to isolated cell-wall material. The ligand to which the protein anchor of L. lactis binds in these cell walls is currently unknown.
The use of a gram-positive, food-grade bacterium, such as Lactococcus lactis, offers significant advantages over the use of other bacteria, such as Salmonella, as a vaccine delivery vehicle. For instance, L. lactis does not replicate in or invade human tissues and reportedly possesses low intrinsic immunity (Norton et al. 1994). Further, mucosal delivered L. lactis that expresses tetanus toxin fragment C has been shown to induce antibodies that protect mice against a lethal challenge with tetanus toxin even if the carrier bacteria was killed prior to administration (Robinson et al. 1997). The killed bacteria still contain recombinant DNA that will be spread into the environment, especially when used in wide-scale oral-immunization programs. However, the uncontrollable shedding of recombinant DNA into the environment may have the risk of being taken up by other bacteria or other microorganisms.